The present invention relates generally to scintillator compositions and related structures and methods. More specifically, the present invention relates to phoswich scintillator compositions having dopant concentrations that vary continuously and sometimes monotonically at a desired minimum rate along a dimension of the scintillator, as well as related devices and methods of fabricating the scintillator compositions using evaporation-based techniques.
Positron emission tomography (PET) is an in-vivo nuclear medicine technique with proven record of accomplishment and with substantial potential to become an even more powerful tool in imaging biological processes in humans and small laboratory animals. With the ever-increasing number of human disease models, particularly in smaller animals such as mice and rats, the ability of high resolution PET technology to contribute important and unique information is apparent to researchers. An important advantage of this emission mode computed tomography technique is that functional data can be obtained non-invasively, allowing each subject to be studied repeatedly in order to monitor the effects of therapeutic interventions over time. With increasing applications of PET continually emerging, PET applications increasingly require systems with excellent spatial resolution, not only because of the small scale of the details to be imaged but also for both detection and quantitation tasks. Thus, there is a demand for economical yet high-performance PET instrumentation that simultaneously optimizes both spatial resolution and detection efficiency, while also offering energy resolution capabilities essential for the improvement of future PET devices.
Unfortunately, in spite of substantial recent improvements in detector technologies having high intrinsic resolution, further improvements are needed. One challenge to improving resolution in PET detectors results from current detectors posing significant challenges in maintaining spatial resolution in the face of parallax errors. Such errors can occur when radionuclide photons are captured at oblique angles using conventional sensors (see, e.g., FIGS. 1A and 1B). In PET the problem is called radial elongation; in SPECT it occurs most prominently with focusing collimators such as pinholes.
Radial elongation error in PET is illustrated with reference to FIG. 1A. For on-axis events, the spatial resolution is determined by the scintillator bar width. For off-axis events, with different depths of interaction, parallax degrades the spatial resolution. For pinhole collimators used in SPECT (e.g., FIG. 1B), different depths of interaction for events near the edge of the field-of-view show resolution degradation due to parallax. For example, parallax errors could be as high as 1.3 mm in SPECT imaging for 140 keV 99mTc gamma ray imaging using a 30° pinhole, and that would completely defeat the 100 μm intrinsic resolution performance achievable with current high resolution photodetector systems. These errors are even higher in PET, where depth of penetration of the 511 keV gamma rays is even higher. The key to improving the spatial resolution performance of current detectors is to develop scintillator structures that minimize parallax errors and couple them to readout sensors with high intrinsic spatial resolution. Thus, a scintillation detector that allows depth-of-interaction (DOI) determination would be an important performance improvement for PET.
Previous attempts have been made to develop DOI scintillators for PET, but all of them had significant performance limitations. The use of decay time to provide DOI has been investigated before using discrete detectors (phoswich detectors). Previous phoswich (“phosphor sandwich”) detectors have included a combination of different and distinct scintillators with dissimilar pulse shape characteristics optically coupled to each other and to (a) common photomultiplier tube(s) (PMT(s)). However, such approaches have thus far been limited in their success, as they typically face problems with detector fabrication, as well as with reflections at interfaces between the various different scintillators that are coupled together. Also, the DOI information is essentially limited to one or two bits, as only a limited number of layers of scintillators with varying decay time could be practically used without appreciable degradation in the signal-to-noise ratios.
Thus, there is a need for improved detectors and PET imaging systems, including those suitable for determining depth of interaction.